That ended Osaka's 16-match Grand Slam winning streak, which included titles at the US Open final in September - when she beat Williams in the final - and at the Australian Open in January.

Osaka was trying to become the first woman to win three consecutive major trophies since Williams grabbed four in a row in 2014-15.

Like Williams, who dropped eight of the match's first 10 games, Osaka couldn't muster a comeback after falling way behind.

"I just feel like there has been a weight on me, kind of," said Osaka, who was seeded No. 1 at a major tournament for the first time.

She fell behind by a set and a break in every match at Roland Garros.

"I could see," Siniakova said, "that she's not so confident like she was."

Osaka said she felt tired and was dealing with a headache.

Katerina Siniakova celebrates her win over No.1 seed Naomi Osaka.

Siniakova never had reached the fourth round in singles in 18 previous Slam appearances. She is better known for her doubles success, winning championships at the French Open and Wimbledon last year and topping the rankings.

She is the first woman ranked No. 1 in doubles to defeat the singles No. 1 in more than 30 years, according to the WTA. The last occasion was when Martina Navratilova beat Steffi Graf at the 1987 US Open.

"I mean, it's incredible. It's amazing. It's the thing I couldn't believe," said Siniakova, who will face 2017 US Open runner-up Madison Keys next. "It was my best tennis."

It decidedly was not Osaka's, who wound up with a hard-to-believe 38 unforced errors; Siniakova made 13.

The first tennis player from Japan to be ranked No. 1 spoke about eyeing a third consecutive major title - and moving halfway to a true Grand Slam.

"It's weird, but I think me losing is probably the best thing that could have happened," the 21-year-old Osaka said. "I think I was overthinking this calendar Slam. For me, this is something that I have wanted to do forever."